The present invention relates to a communication control system which is located between a plurality of terminal apparatuses and a host processing unit in order to send and receive a signal based on an NRZI coding system between them by way of a half duplex communication system.
Hitherto, in the case of communicating data between a host processing unit and terminal apparatus through a line, a modulator/demodulator (hereinafter, abbreviated to a MODEM) is connected for each of the host processing unit and terminal apparatus. The digital data is converted into analog data by the MODEM and thereafter, the analog data is sent to the line, thereby performing the data communication. On the other hand, in the case of receiving data from the line, the analog data is converted into digital data by the MODEM and thereafter, the digital data is sent to the terminal apparatus or host processing unit. In the case of providing a plurality of terminal apparatuses on the side of the terminal apparatus, there is a method whereby a branch circuit to branch the output on a single host processing unit side into inputs on a plurality of terminal apparatus side is provided in the communication control system and the host processing unit is connected to a plurality of terminal apparatus through the branch circuit. There is also a method, wherein branch circuits as many as terminal apparatuses are prepared, the branch circuits are connected to each other, and each branch circuit is connected to a corresponding terminal apparatus. This method is shown, for example, in JP-A 58-24255, wherein a plurality of the terminal apparatuses are connected to a single MODEM through the branch circuits within a range in which a delay time is permitted.
However, nothing is taught with respect to automatic switching between sending and receiving of data. Therefore, no consideration is made with regard to a realization of a high switching speed between the data sending and receiving modes.
On the other hand, for example, in JP-B 60-36147, there has been shown a system in which a signal passing through a line is detected to determine whether the line is in the normal or abnormal condition, thereby switching the mode of operation a selected one of a sending mode, receiving mode, or the like. However, in this system, a signal consisting of a predetermined number (i.e., eight bits) of continuous "0" or "1" bits is detected to thereby discriminate the abnormal or normal condition of the signal on a character unit basis, so that it takes a time for such a discrimination and a high mode switching speed cannot be realized.
In particular, in the case of a half duplex communication system, a high response speed is necessary to improve the sending capability. Therefore, it is necessary to make a switching speed between the sending and receiving modes high.
However, in the foregoing conventional technique, no consideration is made with respect to a high switching speed between the sending and receiving modes in the half duplex communication system. There is a problem such that a response time in the data communication through a communication control system arranged between the host processing unit and the terminal apparatus is prolonged.
On the other hand, in the communication system for sending and receiving data by the half duplex communication system through a communication control system arranged between a plurality of host processing units and a plurality of terminal apparatuses, there has been known a method whereby if the existing host processing unit failed, this failure is displayed and the defective host processing unit is manually switched to a stand-by host processing unit. Therefore, it takes time to switch the mode and so a response time for data sending and receiving becomes long. In addition, no problem will occur if the terminal apparatus and the stand-by host processing unit are controlled by the same transmission control procedure; however, when their transmission control procedures differ, the data communication cannot be performed.